1. Field of the Invention
This invention relates to a process for preparing pyridine-2,3-dicarboxylic acid compounds. More particularly, it relates to a process for preparing pyridine-2,3-dicarboxylic acid compounds which are useful as an intermediate for manufacturing agricultural chemicals and pharmaceuticals.
2. Description of the Prior Art
The pyridine-2,3-dicarboxylic acid compounds are useful intermediates for 2-(2-imidazolin-2-yl)pyridine-3-carboxylic acid derivatives having weeding action as disclosed, for example, in European Patent Application Publication No.A-0041623.
Heretofore, as the method of preparing pyridine-2,3-dicarboxylic acid compounds known are:
(1) Oxidation with nitric acid of quinolines and quinolinols which are synthesized by Skraup reaction from aniline and glycerine with concentrated sulfuric acid and nitrobenzene. (J. Chem. Soc. page 4433, 1956);
(2) Reacting an .alpha., .beta.-unsaturated hydrazone compound and a maleic acid compound in an inert solvent to obtain 1-(substituted amino)-1,4-dihydropyridine-2,3-dicarboxylic acid derivative. Then, heating the resultant derivative to eliminate the substituted amino group in the 1-position. (Japanese Patent Application Unexamined Publication No.246369/1985);
(3) Treating a 1-(substituted amino)-1,2,3,4-tetrahydropyridine-2,3-dicarboxylic acid derivative with acid and/or by heat to convert into a 1,4-dihydropyridine-2,3-dicarboxylic acid derivative, and then oxidizing it. (Japanese Patent Application Unexamined Publication No.47482/1986); and
(4) Oxidizing quinoline with excess hypochlorites in the presence of ruthenium oxide. (Japanese Patent Application Unexamined Publication No.212563/1986)
As the method of synthesizing pyridinemonocarboxylic acid derivatives known is such a method as subjecting ethyl 2-methyl-1,4-dihydronicotinate which is produced by condensation and cyclization of .alpha., .beta.-unsaturated aldehydes such as acrolein and crotonaldehyde with ethyl .beta.-aminocrotonate to oxidation with nitric acid in a mixed acid. (J. Org. Chem. Soc. Vol. 21, page 800, 1956)
However, the above method (1) not only has many reaction processes but also requires drastic oxidation with nitric acid, and involves possible hazards. Also, the pyridine-2,3-dicarboxylic acids, which are apt to cause decarboxylation, result in low yields by the oxidation with nitric acid, and, in addition, produce a large quantity of acidic waste liquid. Thus, the method (1) is not suited to the industrial manufacture of pyridine-2,3-dicarboxylic acids.
The methods (2) and (3) mentioned above have many reaction processes leading to decreased total yields, and require the use of expensive starting materials. Particularly, they require elimination process of the substituted amino group in the intermediate. This decreases the yield and is a problem on resource saving. Therefore, it is difficult to manufacture pyridine-2,3-dicarboxylic acid derivatives industrially by the method (2) or (3).
In the method (4), there are problems that a large excess of the oxidant must be used and that a large quantity of waste liquid is produced requiring expenses for its disposal.